Why is the fourth side of an occlusive dressing left open on a sucking chest wound, particularly if tension pneumothorax develops?

The correct answer is - to allow air and blood to escape.

Explanation:

An occlusive dressing is a dressing that involves water and airtight on the wound or the opening in first aid. It is used to protect the incision or wound to be contaminated.

In the case of the pneumothorax development, the occlusive dressing is sealed with the three-sided but the fourth side is left open so it will allow air to the escape from the pleural space and blood with the prevention of the backflow of air with inspiration.

Thus, the correct answer is - to allow air and blood to escape.